Re: [PATCH 01/14] xfs: document the motivation for online fsck design

Darrick J. Wong
2023-01-18T01:29:59Z

On Wed, Jan 18, 2023 at 12:03:09AM +0000, Allison Henderson wrote:
> On Wed, 2023-01-11 at 11:10 -0800, Darrick J. Wong wrote:
> > On Sat, Jan 07, 2023 at 05:01:54AM +0000, Allison Henderson wrote:
> > > On Fri, 2022-12-30 at 14:10 -0800, Darrick J. Wong wrote:
> > > > From: Darrick J. Wong <djwong@kernel.org>
> > > >
> > > > Start the first chapter of the online fsck design documentation.
> > > > This covers the motivations for creating this in the first place.
> > > >
> > > > Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> > > > ---
> > > >  Documentation/filesystems/index.rst                |    1
> > > >  .../filesystems/xfs-online-fsck-design.rst         |  199
> > > > ++++++++++++++++++++
> > > >  2 files changed, 200 insertions(+)
> > > >  create mode 100644 Documentation/filesystems/xfs-online-fsck-
> > > > design.rst
> > > >
> > > >
> > > > diff --git a/Documentation/filesystems/index.rst
> > > > b/Documentation/filesystems/index.rst
> > > > index bee63d42e5ec..fbb2b5ada95b 100644
> > > > --- a/Documentation/filesystems/index.rst
> > > > +++ b/Documentation/filesystems/index.rst
> > > > @@ -123,4 +123,5 @@ Documentation for filesystem implementations.
> > > >     vfat
> > > >     xfs-delayed-logging-design
> > > >     xfs-self-describing-metadata
> > > > +   xfs-online-fsck-design
> > > >     zonefs
> > > > diff --git a/Documentation/filesystems/xfs-online-fsck-design.rst
> > > > b/Documentation/filesystems/xfs-online-fsck-design.rst
> > > > new file mode 100644
> > > > index 000000000000..25717ebb5f80
> > > > --- /dev/null
> > > > +++ b/Documentation/filesystems/xfs-online-fsck-design.rst
> > > > @@ -0,0 +1,199 @@
> > > > +.. SPDX-License-Identifier: GPL-2.0
> > > > +.. _xfs_online_fsck_design:
> > > > +
> > > > +..
> > > > +        Mapping of heading styles within this document:
> > > > +        Heading 1 uses "====" above and below
> > > > +        Heading 2 uses "===="
> > > > +        Heading 3 uses "----"
> > > > +        Heading 4 uses "````"
> > > > +        Heading 5 uses "^^^^"
> > > > +        Heading 6 uses "~~~~"
> > > > +        Heading 7 uses "...."
> > > > +
> > > > +        Sections are manually numbered because apparently that's
> > > > what everyone
> > > > +        does in the kernel.
> > > > +
> > > > +======================
> > > > +XFS Online Fsck Design
> > > > +======================
> > > > +
> > > > +This document captures the design of the online filesystem check
> > > > feature for
> > > > +XFS.
> > > > +The purpose of this document is threefold:
> > > > +
> > > > +- To help kernel distributors understand exactly what the XFS
> > > > online
> > > > fsck
> > > > +  feature is, and issues about which they should be aware.
> > > > +
> > > > +- To help people reading the code to familiarize themselves with
> > > > the
> > > > relevant
> > > > +  concepts and design points before they start digging into the
> > > > code.
> > > > +
> > > > +- To help developers maintaining the system by capturing the
> > > > reasons
> > > > +  supporting higher level decisionmaking.
> > > nit: decision making
> >
> > Fixed.
> >
> > > > +
> > > > +As the online fsck code is merged, the links in this document to
> > > > topic branches
> > > > +will be replaced with links to code.
> > > > +
> > > > +This document is licensed under the terms of the GNU Public
> > > > License,
> > > > v2.
> > > > +The primary author is Darrick J. Wong.
> > > > +
> > > > +This design document is split into seven parts.
> > > > +Part 1 defines what fsck tools are and the motivations for
> > > > writing a
> > > > new one.
> > > > +Parts 2 and 3 present a high level overview of how online fsck
> > > > process works
> > > > +and how it is tested to ensure correct functionality.
> > > > +Part 4 discusses the user interface and the intended usage modes
> > > > of
> > > > the new
> > > > +program.
> > > > +Parts 5 and 6 show off the high level components and how they
> > > > fit
> > > > together, and
> > > > +then present case studies of how each repair function actually
> > > > works.
> > > > +Part 7 sums up what has been discussed so far and speculates
> > > > about
> > > > what else
> > > > +might be built atop online fsck.
> > > > +
> > > > +.. contents:: Table of Contents
> > > > +   :local:
> > > > +
> > >
> > > Something that I've noticed in my training sessions is that often
> > > times, less is more.  People really only absorb so much over a
> > > particular duration of time, so sometimes having too much detail in
> > > the
> > > context is not as helpful as you might think.  A lot of times,
> > > paraphrasing excerpts to reflect the same info in a more compact
> > > format
> > > will help you keep audience on track (a little longer at least).
> > >
> > > > +1. What is a Filesystem Check?
> > > > +==============================
> > > > +
> > > > +A Unix filesystem has three main jobs: to provide a hierarchy of
> > > > names through
> > > > +which application programs can associate arbitrary blobs of data
> > > > for
> > > > any
> > > > +length of time, to virtualize physical storage media across
> > > > those
> > > > names, and
> > > > +to retrieve the named data blobs at any time.
> > > Consider the following paraphrase:
> > >
> > > A Unix filesystem has three main jobs:
> > >  * Provide a hierarchy of names by which applications access data
> > > for a
> > > length of time.
> > >  * Store or retrieve that data at any time.
> > >  * Virtualize physical storage media across those names
> >
> > Ooh, listifying.  I did quite a bit of that to break up the walls of
> > text in earlier revisions, but apparently I missed this one.
> >
> > > Also... I dont think it would be inappropriate to just skip the
> > > above,
> > > and jump right into fsck.  That's a very limited view of a
> > > filesystem,
> > > likely a reader seeking an fsck doc probably has some idea of what
> > > a fs
> > > is otherwise supposed to be doing.
> >
> > This will become part of the general kernel documentation, so we
> > can't
> > assume that all readers are going to know what a fs really does.
> >
> > "A Unix filesystem has four main responsibilities:
> >
> > - Provide a hierarchy of names through which application programs can
> >   associate arbitrary blobs of data for any length of time,
> >
> > - Virtualize physical storage media across those names, and
> >
> > - Retrieve the named data blobs at any time.
> >
> > - Examine resource usage.
> >
> > "Metadata directly supporting these functions (e.g. files,
> > directories,
> > space mappings) are sometimes called primary metadata.
> > Secondary metadata (e.g. reverse mapping and directory parent
> > pointers)
> > support operations internal to the filesystem, such as internal
> > consistency checking and reorganization."
> Sure, I think that sounds good and helps to set up the metadata
> concepts that are discussed later.
> >
> > (I added those last two sentences in response to a point you made
> > below.)
> >
> > > > +The filesystem check (fsck) tool examines all the metadata in a
> > > > filesystem
> > > > +to look for errors.
> > > > +Simple tools only check for obvious corruptions, but the more
> > > > sophisticated
> > > > +ones cross-reference metadata records to look for
> > > > inconsistencies.
> > > > +People do not like losing data, so most fsck tools also contains
> > > > some ability
> > > > +to deal with any problems found.
> > >
> > > While simple tools can detect data corruptions, a filesystem check
> > > (fsck) uses metadata records as a cross-reference to find and
> > > correct
> > > more inconsistencies.
> > >
> > > ?
> >
> > Let's be careful with the term 'data corruption' here -- a lot of
> > people
> > (well ok me) will see that as *user* data corruption, whereas we're
> > talking about *metadata* corruption.
> >
> > I think I'll rework that second sentence further:
> >
> > "In addition to looking for obvious metadata corruptions, fsck also
> > cross-references different types of metadata records with each other
> > to
> > look for inconsistencies."
> >
> Alrighty, that sounds good
>
> > Since the really dumb fscks of the 1970s are a long ways past now.
> >
> > > > +As a word of caution -- the primary goal of most Linux fsck
> > > > tools is
> > > > to restore
> > > > +the filesystem metadata to a consistent state, not to maximize
> > > > the
> > > > data
> > > > +recovered.
> > > > +That precedent will not be challenged here.
> > > > +
> > > > +Filesystems of the 20th century generally lacked any redundancy
> > > > in
> > > > the ondisk
> > > > +format, which means that fsck can only respond to errors by
> > > > erasing
> > > > files until
> > > > +errors are no longer detected.
> > > > +System administrators avoid data loss by increasing the number
> > > > of
> > > > separate
> > > > +storage systems through the creation of backups;
> > >
> > >
> > > > and they avoid downtime by
> > > > +increasing the redundancy of each storage system through the
> > > > creation of RAID.
> > > Mmm, raids help more for hardware failures right?  They dont really
> > > have a notion of when the fs is corrupted.
> >
> > Right.
> >
> > > While an fsck can help
> > > navigate around a corruption possibly caused by a hardware failure,
> > > I
> > > think it's really a different kind of redundancy. I think I'd
> > > probably
> > > drop the last line and keep the selling point focused online
> > > repair.
> >
> > Yes, RAIDs provide a totally different type of redundancy.  I decided
> > to
> > make this point specifically to counter the people who argue that
> > RAID
> > makes them impervious to corruption problems, etc.
> >
> > This attitude seemed rather prevalent in the early days of btrfs and
> > a
> > certain other filesystem that Shall Not Be Named, even though the
> > btrfs
> > developers themselves acknowledge this distinction, given the
> > existence
> > of `btrfs scrub' and `btrfs check'.
> >
> > However you do have a good point that this sentence doesn't add much
> > where it is.  I think I'll add it as a sidebar at the end of the
> > paragraph.
> >
> > > > +More recent filesystem designs contain enough redundancy in
> > > > their
> > > > metadata that
> > > > +it is now possible to regenerate data structures when non-
> > > > catastrophic errors
> > > > +occur;
> > >
> > >
> > > > this capability aids both strategies.
> > > > +Over the past few years, XFS has added a storage space reverse
> > > > mapping index to
> > > > +make it easy to find which files or metadata objects think they
> > > > own
> > > > a
> > > > +particular range of storage.
> > > > +Efforts are under way to develop a similar reverse mapping index
> > > > for
> > > > the naming
> > > > +hierarchy, which will involve storing directory parent pointers
> > > > in
> > > > each file.
> > > > +With these two pieces in place, XFS uses secondary information
> > > > to
> > > > perform more
> > > > +sophisticated repairs.
> > > This part here I think I would either let go or relocate.  The
> > > topic of
> > > this section is supposed to discuss roughly what a filesystem check
> > > is.
> > > Ideally so we can start talking about how ofsck is different.  It
> > > feels
> > > like a bit of a jump to suddenly hop into rmap and pptrs, and for
> > > "sophisticated repairs" that we havn't really gotten into the
> > > details
> > > of yet.  So I think it would read easier if we saved this part
> > > until we
> > > start talking about how they are used later.
> >
> > Agreed.
> >
> > > > +
> > > > +TLDR; Show Me the Code!
> > > > +-----------------------
> > > > +
> > > > +Code is posted to the kernel.org git trees as follows:
> > > > +`kernel changes
> > > > <
> > > > https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfs-linux.g
> > > > it
> > > > /log/?h=repair-symlink>`_,
> > > > +`userspace changes
> > > > <https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfsprogs-
> > > > dev.
> > > > git/log/?h=scrub-media-scan-service>`_, and
> > > > +`QA test changes
> > > > <https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfstests-
> > > > dev.
> > > > git/log/?h=repair-dirs>`_.
> > > > +Each kernel patchset adding an online repair function will use
> > > > the
> > > > same branch
> > > > +name across the kernel, xfsprogs, and fstests git repos.
> > > > +
> > > > +Existing Tools
> > > > +--------------
> > > > +
> > > > +The online fsck tool described here will be the third tool in
> > > > the
> > > > history of
> > > > +XFS (on Linux) to check and repair filesystems.
> > > > +Two programs precede it:
> > > > +
> > > > +The first program, ``xfs_check``, was created as part of the XFS
> > > > debugger
> > > > +(``xfs_db``) and can only be used with unmounted filesystems.
> > > > +It walks all metadata in the filesystem looking for
> > > > inconsistencies
> > > > in the
> > > > +metadata, though it lacks any ability to repair what it finds.
> > > > +Due to its high memory requirements and inability to repair
> > > > things,
> > > > this
> > > > +program is now deprecated and will not be discussed further.
> > > > +
> > > > +The second program, ``xfs_repair``, was created to be faster and
> > > > more robust
> > > > +than the first program.
> > > > +Like its predecessor, it can only be used with unmounted
> > > > filesystems.
> > > > +It uses extent-based in-memory data structures to reduce memory
> > > > consumption,
> > > > +and tries to schedule readahead IO appropriately to reduce I/O
> > > > waiting time
> > > > +while it scans the metadata of the entire filesystem.
> > > > +The most important feature of this tool is its ability to
> > > > respond to
> > > > +inconsistencies in file metadata and directory tree by erasing
> > > > things as needed
> > > > +to eliminate problems.
> > > > +Space usage metadata are rebuilt from the observed file
> > > > metadata.
> > > > +
> > > > +Problem Statement
> > > > +-----------------
> > > > +
> > > > +The current XFS tools leave several problems unsolved:
> > > > +
> > > > +1. **User programs** suddenly **lose access** to information in
> > > > the
> > > > computer
> > > > +   when unexpected shutdowns occur as a result of silent
> > > > corruptions
> > > > in the
> > > > +   filesystem metadata.
> > > > +   These occur **unpredictably** and often without warning.
> > >
> > >
> > > 1. **User programs** suddenly **lose access** to the filesystem
> > >    when unexpected shutdowns occur as a result of silent
> > > corruptions
> > > that could have otherwise been avoided with an online repair
> > >
> > > While some of these issues are not untrue, I think it makes sense
> > > to
> > > limit them to the issue you plan to solve, and therefore discuss.
> >
> > Fair enough, it's not like one loses /all/ the data in the computer.
> >
> > That said, we're still in the problem definition phase, so I don't
> > want
> > to mention online repair just yet.
> >
> > > > +2. **Users** experience a **total loss of service** during the
> > > > recovery period
> > > > +   after an **unexpected shutdown** occurs.
> > > > +
> > > > +3. **Users** experience a **total loss of service** if the
> > > > filesystem is taken
> > > > +   offline to **look for problems** proactively.
> > > > +
> > > > +4. **Data owners** cannot **check the integrity** of their
> > > > stored
> > > > data without
> > > > +   reading all of it.
> > >
> > > > +   This may expose them to substantial billing costs when a
> > > > linear
> > > > media scan
> > > > +   might suffice.
> > > Ok, I had to re-read this one a few times, but I think this reads a
> > > little cleaner:
> > >
> > >     Customers that are billed for data egress may incur unnecessary
> > > cost when a background media scan on the host may have sufficed
> > >
> > > ?
> >
> > "...when a linear media scan performed by the storage system
> > administrator would suffice."
> >
> That sounds fine to me
>
> > I was tempted to say "storage owner" instead of "storage system
> > administrator" but that sounded a little too IBM.
> >
> > > > +5. **System administrators** cannot **schedule** a maintenance
> > > > window to deal
> > > > +   with corruptions if they **lack the means** to assess
> > > > filesystem
> > > > health
> > > > +   while the filesystem is online.
> > > > +
> > > > +6. **Fleet monitoring tools** cannot **automate periodic
> > > > checks** of
> > > > filesystem
> > > > +   health when doing so requires **manual intervention** and
> > > > downtime.
> > > > +
> > > > +7. **Users** can be tricked into **doing things they do not
> > > > desire**
> > > > when
> > > > +   malicious actors **exploit quirks of Unicode** to place
> > > > misleading names
> > > > +   in directories.
> > > hrmm, I guess I'm not immediately extrapolating what things users
> > > are
> > > being tricked into doing, or how ofsck solves this?  Otherwise I
> > > might
> > > drop the last one here, I think the rest of the bullets are plenty
> > > of
> > > motivation.
> >
> > The doc gets into this later[1], but it's possible to create two
> > entries
> > within the same directory that have different byte sequences in the
> > name
> > but render identically in file choosers.  These pathnames:
> >
> > /home/djwong/Downloads/rustup.sh
> > /home/djwong/Downloads/rus<zero width space>tup.sh
> >
> > refer to different files, but a naïve file open dialog will render
> > them
> > identically as "rustup.sh".  If the first is the Rust installer and
> > the
> > second name is actually a ransomware payload, I can victimize you by
> > tricking you into opening the wrong one.
> >
> > Firefox had a whole CVE over this in 2018:
> > https://bugzilla.mozilla.org/show_bug.cgi?id=1438025
> >
> > xfs_scrub is (so far) the only linux filesystem fsck tool that will
> > warn
> > system administrators about this kind of thing.
> >
> > See generic/453 and generic/454.
> >
> > [1] https://djwong.org/docs/xfs-online-fsck-design/#id108
> >
> hmm ok, how about:
>
> 7. Malicious attacks may use uncommon unicode characters to create file
> names that resemble normal files, which may go undetected until the
> filesystem is scanned.

They resemble *other filenames* in the same directory, normal or
otherwise.

Note that xattrs have the same problem -- a listing of attrs will show
two names that render identically but map to different things. There's
less double-click danger there, at least.

Another class of unicode problem is that you can use directional
controls to spoof file extensions. The sequence:

pu<right to left>txt.pl

renders as "pulp.txt" if you're not careful, but file managers think
it's actually a perl script file! Granted, nobody should allow
execution of random a-x downloaded scripts.

There are enough weird twists to this sort of deception that I left #7
worded as broadly as I needed.

--D

>
> ?
>
> > > > +
> > > > +Given this definition of the problems to be solved and the
> > > > actors
> > > > who would
> > > > +benefit, the proposed solution is a third fsck tool that acts on
> > > > a
> > > > running
> > > > +filesystem.
> > > > +
> > > > +This new third program has three components: an in-kernel
> > > > facility
> > > > to check
> > > > +metadata, an in-kernel facility to repair metadata, and a
> > > > userspace
> > > > driver
> > > > +program to drive fsck activity on a live filesystem.
> > > > +``xfs_scrub`` is the name of the driver program.
> > > > +The rest of this document presents the goals and use cases of
> > > > the
> > > > new fsck
> > > > +tool, describes its major design points in connection to those
> > > > goals, and
> > > > +discusses the similarities and differences with existing tools.
> > > > +
> > > > ++---------------------------------------------------------------
> > > > ----
> > > > -------+
> > > > +|
> > > > **Note**:
> > > >
> > > >     |
> > > > ++---------------------------------------------------------------
> > > > ----
> > > > -------+
> > > > +| Throughout this document, the existing offline fsck tool can
> > > > also
> > > > be     |
> > > > +| referred to by its current name
> > > > "``xfs_repair``".                        |
> > > > +| The userspace driver program for the new online fsck tool can
> > > > be         |
> > > > +| referred to as
> > > > "``xfs_scrub``".                                          |
> > > > +| The kernel portion of online fsck that validates metadata is
> > > > called      |
> > > > +| "online scrub", and portion of the kernel that fixes metadata
> > > > is
> > > > called  |
> > > > +| "online
> > > > repair".
> > > > |
> > > > ++---------------------------------------------------------------
> > > > ----
> > > > -------+
> >
> > Errr ^^^^ is Evolution doing line wrapping here?
> >
> > > Hmm, maybe here might be a good spot to move rmap and pptrs?  It's
> > > not
> > > otherwise clear to me what "secondary metadata" is.  If that is
> > > what it
> > > is meant to refer to, I think the reader will more intuitively make
> > > the
> > > connection if those two blurbs appear in the same context.
> >
> > Ooh, you found a significant gap-- nowhere in this chapter do I
> > actually
> > define what is primary metadata.  Or secondary metadata.
> >
> > > > +
> > > > +Secondary metadata indices enable the reconstruction of parts of
> > > > a
> > > > damaged
> > > > +primary metadata object from secondary information.
> > >
> > > I would take out this blurb...
> > > > +XFS filesystems shard themselves into multiple primary objects
> > > > to
> > > > enable better
> > > > +performance on highly threaded systems and to contain the blast
> > > > radius when
> > > > +problems happen.
> > >
> > >
> > > > +The naming hierarchy is broken up into objects known as
> > > > directories
> > > > and files;
> > > > +and the physical space is split into pieces known as allocation
> > > > groups.
> > > And add here:
> > >
> > > "This enables better performance on highly threaded systems and
> > > helps
> > > to contain corruptions when they occur."
> > >
> > > I think that reads cleaner
> >
> > Ok.  Mind if I reword this slightly?  The entire paragraph now reads
> > like this:
> >
> > "The naming hierarchy is broken up into objects known as directories
> > and
> > files and the physical space is split into pieces known as allocation
> > groups.  Sharding enables better performance on highly parallel
> > systems
> > and helps to contain the damage when corruptions occur.  The division
> > of
> > the filesystem into principal objects (allocation groups and inodes)
> > means that there are ample opportunities to perform targeted checks
> > and
> > repairs on a subset of the filesystem."
> I think that sounds cleaner
>
> >
> > > > +The division of the filesystem into principal objects
> > > > (allocation
> > > > groups and
> > > > +inodes) means that there are ample opportunities to perform
> > > > targeted
> > > > checks and
> > > > +repairs on a subset of the filesystem.
> > > > +While this is going on, other parts continue processing IO
> > > > requests.
> > > > +Even if a piece of filesystem metadata can only be regenerated
> > > > by
> > > > scanning the
> > > > +entire system, the scan can still be done in the background
> > > > while
> > > > other file
> > > > +operations continue.
> > > > +
> > > > +In summary, online fsck takes advantage of resource sharding and
> > > > redundant
> > > > +metadata to enable targeted checking and repair operations while
> > > > the
> > > > system
> > > > +is running.
> > > > +This capability will be coupled to automatic system management
> > > > so
> > > > that
> > > > +autonomous self-healing of XFS maximizes service availability.
> > > >
> > >
> > > Nits and paraphrases aside, I think this looks pretty good?
> >
> > Woot.  Thanks for digging in! :)
> >
> Sure, no problem!
>
> > > Allison
> > >
>